Various permeable or semipermeable membranes are known to be used in a variety of fluid separations. These membranes can offer, inter alia, significant cost savings over other fluid separation means, such as adsorption and distillation means. The effectiveness of the membranes in fluid separation, however, is highly dependent on the permeability and selectivity of the membranes. As the increased permeability requirement in the recent years has resulted in the movement of membrane art in the direction of reducing the thickness of membranes, the strength and film forming characteristics of membrane materials have also become increasingly important. The strength and film forming characteristics play an important role in producing very thin membranes.
At the present time, fluid separation membranes are known to be made from a wide variety of polymeric materials. Of these polymeric materials, poly(phenylene oxide) polymer or chemically modified poly(phenylene oxide) polymer, such as sulfonated poly(phenylene oxide), is most attractive as a membrane forming material because of the good combination of permeation and separation characteristics of these materials and because of the ready availability of poly(phenylene oxide) polymer. Indeed, such desirability of poly(phenylene oxide) based polymer materials has led to the development of a variety of poly(phenylene oxide) polymer based membranes.
Chemical Abstract No. 236575Y, volume 112, "Hollow Poly(phenylene oxide) Fibers and Process for Manufacturing Them" by M. Pechocova et al, Czech. CS 263,672, shows an asymmetric membrane, i.e., hollow fibers, useful for reverse osmosis, micro- and ultra-filtration, dialysis and other separation processes. The asymmetric membrane is prepared by spinning a solution containing poly(2,6-dimethyl-1,4-phenylene oxide) polymer or sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) polymer having a molecular weight equal to or greater than 74,000 through a spinneret with ring-shaped holes.
J. Smid et al. in the Journal of Membrane Science, Vol 64, p. 121-129, 1991, "The Formation of Asymmetric Hollow Fiber Membranes for Gas Separation, Using PPE of Different Intrinsic Viscosities" disclose asymmetric hollow fiber membranes prepared by using a range of intrinsic viscosities of poly (2,6-dimethyl-1,4-phenylene oxide). The use of high intrinsic viscosity poly (2,6-dimethyl-1,4-phenylene oxide) polymer is indicated to be helpful in preparing asymmetric membranes, i.e., hollow fibers, with improved properties.
U.S Pat. No. 3,259,592 discloses resins and membranes made from sulfonated poly(phenylene oxide) polymer. The sulfonated poly(phenylene oxide) polymers disclosed are produced by a sulfonation process that is substantially heterogeneous in nature. The sulfonated polymer precipitates instantaneously on the addition of the sulfonating agent, HClSO.sub.3, to the solution containing precursor poly(phenylene oxide) polymer in a halogenated solvent, and the sulfonation is completed by the further addition of the sulfonating agent to the slurry of the partially sulfonated product.
Preparation of sulfonated poly(phenylene oxide) membranes for reverse osmosis applications is described in Research and Development Progress Report No. 697, Contract No. 14-30-2627 to the U.S. Dept. of Interior, entitled "Development of Poly(phenylene oxide) Membranes" by Chludzinski et al., 1971. Commercial poly(phenylene oxide) polymer that had an intrinsic viscosity of about 0.45 dl/g was utilized to prepare sulfonated polymeric membrane forming materials.
The commercial application of modified poly(phenylene oxide) based membranes, however, is frequently hindered by their limited separation characteristics and/or strength in fluid separations, more particularly in gas separation applications. Often, the membranes produced are fragile, brittle and prone to defects. Commercial membranes normally could not be produced with consistent properties.
Therefore, there is a genuine need and desire in the art for modified poly(phenylene oxide) polymer based membranes having enhanced selectivity, permeability and strength, which are capable of being used in commercial fluid separation operations, more particularly in commercial gas separation operations. Moreover, it is desirable to determine better ways to produce modified poly(phenylene oxide) polymer based membrane materials having the desired consistent properties, i.e., permeability, selectivity, strength and film forming characteristics.